Technical data

Heat transfer takes place via the heated shelves by direct contact
with the bottom of the vessel and/or by convection via the shelf
and vessel or product.
At the beginning of sublimation the transfer of heat is very effective
from the wall of the vessel to the frozen product. However, soon
an area develops which is ice-free, porous and dried and has a
corresponding temperature gradient between the wall of the vessel
and the product. The poor heat conductivity of the already dried
product can lead to an increase in temperature of the ice core. If
the core temperature rises above the solidification temperature,
the product begins to thaw. This applies especially to
inhomogeneous products and to great layer thicknesses. During
this drying phase it is important to regulate the heat supply and
control temperature and pressure precisely.
2.5. Final drying
The final pressure in the drying chamber depends on the ice
condenser temperature according to the vapor pressure curve
above ice :
e. g. 1.030 mbar correspond to -20°C
0.370 mbar correspond to -30°C
0.120 mbar correspond to -40°C
0.040 mbar correspond to -50°C
0.011 mbar correspond to -60°C
The unit is in operating condition if the temperature of the ice
condenser is lower than -50°C and the pressure is lower than
0.120 mbar.
The final pressure measured by the vacuum sensor when there is
no product in the unit and its corresponding ice temperature value
is mainly determined by the
warmest place of the ice on the ice
condenser chamber. Moreover, this value is affected by residues
or parts of solvents in the product with a higher vapor pressure.
Operating Manual Freeze Dryer ALPHA 1-4 LSC / ALPHA 2-4 LSC General Information on Freeze Drying • 17